The present invention relates to a method of operating an internal combustion engine, in particular, a method of operating an internal combustion engine of a motor vehicle, in which fuel is injected directly into a combustion chamber of the engine during an intake phase in a first mode of operation or during a compression phase in a second mode of operation, and in which exhaust gas is sent to a catalytic converter. The present invention also relates to a control unit for an engine, and in particular, to a control unit for an engine of a motor vehicle.
Conventionally, in direct gasoline injections, fuel is injected into the combustion chamber of the engine during the intake phase in homogeneous operation or during the compression phase in stratified charge operation. Homogeneous operation may be provided for full-load operation of the engine, while stratified charge operation is suitable for idling and partial load operation. Such a direct injection engine is switched between these modes of operation as a function of the required torque, for example.
For conversion of exhaust gases in the catalytic converter with the engine described above, the catalytic converter typically has a specified operating temperature.
The engine may be operated at an operating point at which the exhaust gas temperature is low. Such an operating point occurs in idling operation, for example, when the engine is operated at a low idling speed.
In such idling operation, very little fuel is injected into the engine. This may result in the catalytic converter cooling down because of the low resulting exhaust gas temperature. Conversion and thus purification of the exhaust gases of the engine are then no longer guaranteed.
An object of the present invention is to provide a method of operating an internal combustion engine with which adequate purification of the exhaust gases is guaranteed even at operating points having a low exhaust gas temperature.
A temperature difference between an actual exhaust gas temperature and a setpoint exhaust gas temperature is determined at an operating point having a low exhaust gas temperature, and at least one additional injection is implemented after combustion, as a function of the temperature difference. This may be performed by a control unit of an internal combustion engine.
Due to the additional injection, a combustible mixture is produced in the area of the exhaust bend or the catalytic converter. Due to the fact that the exhaust bend or the catalytic converter is still hot, this mixture is ignited and burned. The heat generated in this way prevents the catalytic converter from cooling down. The conversion capacity of the catalytic converter is maintained and purification of the exhaust gases is thus guaranteed even at operating points having a low exhaust gas temperature.
The dependence of the additional injection on the temperature difference between the actual exhaust gas temperature and the setpoint exhaust gas temperature guarantees that there is only a minimal increase in consumption due to heating of the catalytic converter. This is equivalent to a low additional production of exhaust gas and/or pollutants due to heating of the catalytic converter.
According to an embodiment of the present invention, a minimum lambda value of the exhaust gas is defined, and the at least one additional injection after combustion is limited as a function of this minimum lambda value. This guarantees that the lambda value of the exhaust gas does not become too rich and thus does not have an excessively high pollutant content.
According to a second embodiment of the present invention, a fixed lambda value of the exhaust gas is specified, and the at least one additional injection after combustion is implemented as a function of this fixed lambda value, and the start of injection of the additional injection is altered. In this way, a specified lambda value of the exhaust gas is guaranteed. In order for the catalytic converter not to become overheated in any case, the start of injection of the additional injection may be varied if necessary.
It is also advantageous if the start of injection is varied as a function of the actual exhaust gas temperature. This makes it possible to achieve a control and/or regulation of the start of injection.
In another embodiment of the present invention, the number and/or times of the additional injections are selected so that the operating temperature of the catalytic converter does not drop below the setpoint and/or the catalytic converter is not overheated. The number and/or times may be determined in advance. Alternatively or additionally, it is possible to control and/or regulate the number and/or times as a function of the instantaneous temperature of the catalytic converter.
In addition, it is also possible for the temperature of the catalytic converter to be measured or modeled and for the additional injections to be implemented only when the temperature drops below a limit temperature. This permits fuel savings and a reduction in pollutant emissions.
The present invention may be applied in idling and/or in the second mode of operation of the engine. Idling is an operating point at which a low exhaust gas temperature may occur and at which the catalytic converter may therefore drop below its operating temperature. Idling is usually implemented in stratified charge operation in an internal combustion engine having direct injection. The method according to the present invention may therefore be used in stratified charge operation and in idling of the engine.
The method according to the present invention may be implemented using a control element provided for a control unit of an internal combustion engine in a motor vehicle. A program capable of running on a computer, in particular on a microprocessor, and suitable for execution of the method according to the present invention may be stored on this control element. An electric memory medium such as a read-only memory or a flash memory in particular may be used as the control element.